1. Field of the Invention
The present invention relates to a vehicle steering system, a vehicle including the steering system, and a method for turning at least one wheel of a vehicle.
2. Background of the Invention
In recent years, a steer-by-wire system has been suggested in which a steering shaft coupled to a steering wheel is mechanically separated from a rudder mechanism that turns turning wheels, and the system electrically controls a steering motor provided in the system (for example, see Japanese Unexamined Patent Application Publication No. 2005-29016). Japanese Unexamined Patent Application Publication No. 2005-29016 is incorporated herein by reference in their entirety.
In addition to the steer-by-wire system, an active steering system has been also suggested in which the controlling amount of the rudder angle of turning wheels is variable with respect to the operating amount applied by a driver with a steering wheel. Such a steering system determines a target rudder angle of the turning wheels and turns the turning wheels corresponding to the determined target rudder angle.
The steering system determines a target rudder angle δf, on the basis of the sum of a proportional term that is proportional to a steering angle θh and a derivative term that is proportional to a steering angular speed θh′ (time differential of the steering angle θh), for example, as follows:δf=K0(V)·θh+K1(V)·θh′where V is a vehicle speed, K0(V) is a proportional control gain determined in accordance with the vehicle speed V, and K1(V) is a differential control gain determined in accordance with the vehicle speed V. Determining the target rudder angle δf on the basis of the sum of the proportional term and the differential term. In this way, the response of the vehicle corresponding to the steering operation can be improved.
Determining the target rudder angle can improve the response of vehicle behavior, however, the convergence of the behavior may be degraded. In particular, if the differential control gain K1(V) is set to a large value, then the response of the vehicle behavior to the steering operation performed by the driver can be further improved, whereas the convergence and stability of the vehicle behavior are degraded.
To be more specific, since the differential term is included, positive and negative values of the differential term may be reversed, for example, when the driver turns the steering wheel in a reverse direction. As a result, the target rudder angle may be markedly changed without a driver's intent, degrading the convergence and stability of the vehicle behavior.